Somatic cell genetic analysis has permitted the detection, characterization and genetic localization of a number of mammalian cellular genes which participate in endogenous type C virus expression. Three general classes of genes have been studied: (1) endogenous cellular DNA sequences homologous to type C viral RNA; (2) integration sites of retroviruses in mammalian chromosomes; (3) restriction genes which delimit virus replication in certain animal species. Examples of each of these gene classes have been described largely in human and cat genomes. The concept and technology of the allozyme genetic signature has been developed and applied to species and type identification of culture cells, and also as markers for studies of maternal vs. fetal tissue during embryonic development. A biochemical genetic map of the cat including gene-enzyme loci, retrovirus loci, and genes encoding cell surface antigens is under construction.